Microwave moisture sensing system and method

ABSTRACT

An apparatus for measuring the amount of moisture in a sample which is generally formed of a microwave energy source, a microwave radiating element connected to the source, a microwave energy receiving element, and a microwave energy indication connected to the receiving element. The sample is positioned between the radiating and receiving elements and a structure is provided for changing the energy transmission path through the sample by at least one-half wavelength of the microwave energy. Such means may take the form of a frequency modulator connected to the source, a dielectric disc in the transmission path, or a structure for continuously changing the distance between the radiating element and the sample or combination of these means.

United States Patent Busker et a].

[451 Sept. 26, 1972 [54] MICROWAVE MOISTURE SENSING 2,457,695 12/1948Liskow ..324/58.5 A

SYSTEM AND METHOD Primary Examiner-Michael J. Lynch [72] k311 f' f 'g iff Attorney-Hill & Hill and Hill, Sherman, Meroni,

0s e 01 Gross & Simpson [73] Assignee: Beloit Corporation, Beloit, Wis.[22] Filed: Aug. 3, 1970 [57] f ABSTRACT f An apparatus or measuring theamount 0 moisture in [21] Appl. No.. 67,643 a sample which is generallyformed of a microwave Related Us. Application Data energy source, amicrowave radiating element connected to the source, a microwave energyreceiving [62] Division of Ser. No. 679,325, Oct. 31, 1967, element, anda microwave energy indication con- Pat. No. 3,599,088. nected to thereceiving element. The sample is positioned between the radiating andreceiving elements [52] 0.8. CI. ..324/58.5 A and a structure isprovided for g g h gy 51 Int. Cl. ..'...G0'lr27/04 transmission Pthrough the sample y at least [58] Field of Search ..324/5s, 58.5 halfWavelength of the microwave snsrgy- Such means may take the form of afrequency modulator con- [56] References Cited nected to the source, adielectric disc in the transmis sion path, or a structure forcontinuously changing the UNITED STATES PATENTS distance between theradiating element and the sam- 3,079,551 2/1963 Walker ..324/58.5 A pleor cmbmann these means 3,460,031 8/1969 Evans ..324/58.5 A 1 Claim, 16Drawing Figures mbf i n .fi f f f 2. 1? l d I 2 /Z a /9 Z Mapuhfiok//V7E&RA70

' establish a standing wave due to reflection of energy MICROWAVEMOISTURE SENSING SYSTEM AND METHOD CROSS REFERENCE TO RELATEDAPPLICATIONS This application is a division of application Ser. No.679,325 filed Oct. 31, 1967- entitled Microwave Moisture Sensing SystemIncluding Means To Continuously Change The Transmission Path of theMicrowave Energy, now US. Pat. No. 3,599,088 which issued on Aug.10,1971.

BACKGROUND OF THE INVENTION moisture content of any material, whetherliquid, semiliquid, or solid.

2. Description of the Prior Art The method for measuring moisturecontent of material which consists of placing a sample between twomicrowave horns which are connected to a microwave generator and amicrowave detector is well known in the prior art. However, thistechnique is applicable only when the position of the sample between thehorns and the distance between the horns can be accurately maintained.It has been found that there is a drastic effect upon the moisturesignal due to the position of the sample with respect to the microwavehorns.

It is well established that a dielectric sample placed between amicrowave sending and receiving horn will from the faces of thedielectric. If the receiving horn is assumed to have no reflection, thenwithout the sample present, no standing wave exists. However,'when thesample is placed between the horns, a standing wave is established whichis dependent upon the amount of reflected energy from the surface andthe position of the sample.

When the moisture content of a paper web, for example, is beingmeasured, the position of the web cannot be maintained fixed withrespect to the horns without a considerable amount of difficulty beingencountered. In the measurement of moisture of a paper web, it isnecessary to scan across the paper web to determine the moisture contentof the entire width thereof. Such a scanning of the paper web, whichrequires either movement of the horns, or movement of the web withrespect to the horns, usually results in some change in the distancebetween the horns and the web. As a result of such a change, thestanding wave which exists will affect the amount of energy which isreflected from the surface of the sample. Since the measurement of suchreflected energy or the measurement of the transmitted energy isemployed in the determination of the moisture content within the sample,such a change of position of the sample will result in erroneousreadings indicative of the moisture content.

In the manufacture of paper, the moisture content of a paper web must bedetermined during the manufacturing process. That is, when the paper webis being dryed, for instance, it may be desirable to determine themoisture content therein. Since such drying of the paper web is effectedon drying rolls, the measurement of the moisture content mustnecessarily be effective at some point intermediate the drying rollswhere the measuring apparatus can be easily inserted and mounted withrespect to the traveling paper web. However, a traveling paper web at apoint intermediate a pair of drying rolls normally has a component ofmovement transverse to the plane thereof. That is, between the dryingrolls, a traveling paper web may flutter. Such movement of the paper webwould be towards and away from the microwave horns positioned onopposite side thereof employed for measuring the moisture contentthereof. Such movement with respect to the microwave horns, as discussedabove, would produce erroneous readings with respect to the moisturecontent thereof. Therefore, a need exists for a system which eliminatesthe position sensitivity of a sample.

SUMMARY OF THE INVENTION It is, therefore, an object of the presentinvention to provide an apparatus for measuring the amount of moisturein a sample which eliminates the difficulty of the standing waveresulting from the position of a sample.

It is another object of the present invention to provide an apparatusfor measuring the moisture content in a sample which eliminates thedifficulty of the changing reflection from the sample surface as itsspacing between the microwave horns changes.

These and other objects are realized by the structure of the presentinvention which generally includes a microwave energy radiating elementpositioned to impinge microwave energy on the sample, a microwave energyreceiving element positioned to receive microwave energy transmittedthrough the sample, and means for continuously changing the transmissionpath from the radiating element to the sample as measured in wavelengths of the microwave energy at least onehalf wavelength of themicrowave energy.

A specific feature of the present invention resides in the provision ofa frequency modulator connected to source of microwave energy, whichmodulator sweeps the frequency of the source through a predeterminedrange which effectively changes the transmission path from the radiatingelement to the sample at least onehalf wavelength of the microwaveenergy.

Another feature of the present invention resides in the provision ofmeans for continuously changing the physical distance between theradiating element and the sample, with the change in the physicaldistance being sufficient to provide a change of at least one-half wavelength in the transmission path of the microwave energy therebetween.

Still another feature of the present invention resides in the provisionof a dielectric disc of varying thickness mounted between the radiatingelement and the sample and means for rotating the disc at apredetermined rate to present a continuously changing transmission paththrough the disc to the microwave energy impinging on the sample.

These and other objects, features and advantages of the presentinvention will be more fully realized and understood from the followingdetailed description when taken in conjunction with the accompanyingdrawings wherein:

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a diagrammatic view of anapparatus for measuring the amount of moisture in a sample andillustrates the transmission of energy to and from the sample;

FIG. 2 is a diagrammatic showing of an apparatus for measuring theamount of moisture in a sample and constructed in accordance with theprinciples of the present invention;

I FIG. 3 is a diagrammatic showing of another embodiment of the presentinvention for measuring the amount of moisture in a sample;

FIG. 4 is a diagrammatic showing of still another embodiment of thepresent invention;

FIG. 5 is a diagrammatic showing of another embodiment of the presentinvention;

FIG. 6 is a diagrammatic showing of an apparatus for measuring theamount of moisture in a sample which employs apparatus for amplifyingthe transmitted energy passing through the sample;

FIG. 7 is a diagrammatic showing of still another embodiment of thepresent invention for measuring the moisture content in a sample inwhich the moisture content is relatively low;

FIG. 8 is an end view of the measuring system illustrated in FIG. 7;

FIG. 9 is still another embodiment of the apparatus of the presentinvention for measuring the moisture content of a sample wherein themoisture content is relatively low;

FIG. 10 is a diagrammatic showing of an apparatus for measuring themoisture content of a sample and -constructed as a travelling waveresonator for overcoming the low loss encountered in relatively dry pFIG. 11 is a side elevational view of an apparatus for mounting a pairof microwave horns on opposite sides of a sample and disposed forrelative movement therebetween;

FIG. 12 is a front view of the apparatus illustrated in FIG. 11;

FIG. 13 is an elevational view of a structural unit employed forhandling a slurry and which is adapted to be supported between a pair ofmicrowave horns for determining the amount of moisture content therein;

FIG. 14 is a side elevational view of the structural unit illustrated inFIG. 13;

FIG. 15 is an elevational view of one of the components of thestructural unit illustrated in FIGS. 13 and 14; and

FIG. 16 is an elevational view of the other component of the structuralunit illustrated in FIGS. 13 and 14 and disposed for assembly with thecomponent illustrated in FIG. 15.

Like reference numerals throughout the various views of the drawings areintended to designate the same or similar structures.

DESCRIPTION OF THE PREFERRED EMBODIMENTS With reference to FIG. 1 of thedrawings, there is shown diagrammatically an apparatus for measuring themoisture content of a paper web. As shown therein, a microwave energysource 15 is connected to a microwave energy transmitting horn 16 whichis disposed for transmitting microwave energy designated P, toward asample 17, such as a paper web. The energy P, transmitted through thesample is detected or received by a microwave energy receiving horn 18which is connected to an indicating device 19. The reflected energy P,is transmitted back toward the transmitting horn 16. Determination ofthe moisture content of the sample 17 is provided by a measure of theamount of energy P, which is detected by the receiving horn 18.

When the sample 17 is placed between the horns 16 and 18, a standingwave is established which is dependent upon the amount of reflectedenergy from the surface and the position of the sample with respect'tothe horns. Movement of the sample 17 through a quarter wavelengthproduces conditions of cancellation and addition of the microwave energywhich appears as minimum and maximum reflected power. Minimums ormaximums of the standing wave are found to appear every one-halfwavelength. Because of the difficulty in maintaining the position of thesample fixed with respect to the horns, the present inventioncontemplates a system which eliminates the error caused by the standingwave which changes with changes in the sample position.

In order to eliminate the problem of sample position sensitivity, thepresent invention contemplates the technique of sweeping the frequencyof the signal source 15 thus obtaining an average DC reading of thevarying transmitted power. If certain minimum conditions are met, theaverage reading is independent of sample position and serves as a veryuseful measurement of absorption of the microwave energy by the moisturewithin the sample l7.'

The condition which must be met is that the broad band frequencymodulation of change of microwave frequency will be of sufficient changeto produce at least a one-half wavelength change in transmission pathbetween the transmitting horn l6 and the sample 17 to insure passingfrom a minimum to a maximum of reflected power. It is not necessary tobegin at any given condition of reflected power, if the change is atleast one-half wavelength in the transmission path.

If, for example, a center frequency of 22,250 gigacycles (1.35 cm) isgenerated by the source 15, a total sweep of l gigacycle will besufficient to provide a onehalf wavelength change in the transmissionpath between the transmitting horn 16 and the sample 17. The total sweepof l gigacycle corresponds to wavelengths of 1.32 to 1.38 cm. To shiftone-half wavelength, the minimum distance to the sample 17 from the horn16 may be calculated as follows:

wherein A is the wavelength of the signal;

= 0.06 cm(Af= ICC);

L= (M2) (1/AA)= (0.675/0.06) 1 L28 cm;

wherein L is the minimum distance between the receiving horn and thesample or the transmitting horn and the sample.

In practice, either greater separation distance or greater sweep offrequency appears necessary to obtain a good average value.

In practice, either greater separation distance or greater sweep offrequency appears necessary to obtain a good average value. Forinstance, a frequency modulation of at least 2 or 3 gigahertz at 22gigahertz center frequency appears necessary.

It has also been found that sample position sensitivity may besignificantly reduced by the method of slanting the sample web withrespect to the microwave horns 16 and 18.

The electronic sweep of klystrons is limited to about 50 megahertz. Toobtain a minimum of one-half wavelength shift would require a separationdistance of approximately 1 1 feet. Therefore, the use of a klystronwould probably not prove practical. However, devices are available, suchas backward wave oscillators, that will provide a sweep of the frequencyof approximately 6 or 8 gigahertz.

One embodiment of applicants invention for providing such a sweep of thesource frequency is illustrated in FIG. 2 of the drawings. As showntherein, the source may take the form of a microwave oscillator which iscontrolled by a modulator to provide a frequency sweep of the signal atthe output thereof. The output of the oscillator 15 is connected to thetransmitting microwave horn 16 which transmits microwave energy throughthe sample 17 to the receiving horn 18. The horn 18 is connected to arectifier 21 to an integrator 22. The continually changing frequencysignal which is detected by the horn 18 is integrated by the integrator22 to provide an average thereof as a DC signal which is supplied to anindicating device 19. The average value of the signal detected by thereceiving horn 18 provides a measurement of the moisture content withinthe sample 17 which is independent of the position of the sample 17.

Another embodiment of applicants invention is illustrated in FIG. 3 ofthe drawing. As shown in FIG. 3, the oscillator 15 provides a signal offixed frequency to the transmitting horn 16. Disposed between thetransmitting horn 16 and the sample 17 is a chopper in the form of awheel or disc of varying thickness made of a high dielectric low lossmaterial. One face 23a of the disc 23 is curved and the other face 23bthereof is flat to present a varying thickness to the energy transmittedfrom the horn 16 toward the sample 17 as the disc 23 is rotated. Thedisc 23 is mounted on a rotatable shaft which is connected to a motor 25for rotation of the disc 23. Rotation of the disc 23 causes a change inthe transmission path length of one-half wavelength which is achieved byvariation of the dielectric material thickness during such rotation. Theflat surface 23b of the disc 23 minimizes and maintains constant thereflections to the transmitting horn 16. The signal detected by thereceiving horn 18 is integrated by the integrator 22 to provide anaverage of the detected signal. The average value of such signal issupplied in the form of a DC voltage to the indicator 19.

Still another embodiment of applicants invention is illustrated in FIG.4 which employs a device for maintaining the sample 17 fixed withrespect to the horns l6 and 18. As shown therein, the source 15 ofmicrowave energy is modulated by the modulator 20, and the modulatedsignal at the output of the source 15 is connected to the transmittinghorn 16. As in the previously described embodiments, the receiving horn18 is connected to the rectifier 21 to the integrator 22, which is inturn connected to the indicating device 19. The source 15 is connectedto the horn 16 and the horn 18 is connected to the integrator 22 byrespective wave guides which are diagrammatically illustrated by thesingle line connections in the figure. In order to maintain the hornspacing rigid, a mechanical support 26 is provided between therespective wave guides. The sample 27 is supported on a Teflon shoe 27,which is in turn supported on the receiving horn 18. In this manner, thespacing between the horns 16 and 18 is rigidly maintained by means ofthe mechanical support 26 and the position of the sample 17 is rigidlymaintained with respect to the horn 18.

FIG. 5 illustrates another embodiment of applicants invention whichemploys a method of measurement based upon the dielectric constant(reflection) of the sample 17, rather than the loss factor (absorption)of the sample 17. At any air to dielectric interface, a reflection ofincident energy occurs. The amount of reflected energy or the reflectionfactor is a function of the dielectric constant of the material, asexpressed by the following, wherein r is the amount of reflected energyand e is the dielectric constant of the material:

As shown in FIG. 5, the modulator 20 is connected to the source ofmicrowave energy 15 and modulates the output thereof which is connectedto a wave guide 16a to the transmitting horn 16. A portion of the energywhich is transmitted by the horn 16 passes through the sample 17 and isabsorbed on a non-reflecting surface 28. The remaining portion of theenergy transmitted by the horn 16 is reflected from the sample 17 andreturns to the horn 16. A directional coupler 29 is connected betweenthe wave guide 16a and a wave guide 29a for coupling the reflectedenergy received by the horn 16 to the wave guide 29a. The wave guide 29ais connected through the rectifier 21 to the integrator 22, which is inturn connected to the microwave indicator. 19 which provides anindication of the reflected energy from the sample 17. Since thedielectric constant of the sample 17 varies with the moisture content,measurement of such reflected energy provides an indication of theamount of moisture content within the sample 17.

A difficulty encountered in applying the frequency modulation method tomeasuring moisture in a sample occurs when the moisture content isrelatively low, for instance below 30 percent. With high moisturecontent, an adequate signal to noise ratio is obtained when the horns 16and 18 are positioned perpendicular to the plane of the sample 17 andthe energy is passed through the sample only once. At low moisturecontent, such as to ISpercent moisture, the attenuation attained bypassing the energy once or even twice through the sheet is not adequatefor practical use. It is possible to obtain an adequate attenuationlevel by passing the energy through the sheet several times.

Such an arrangement is illustrated in FIG. 6, wherein the frequencymodulator is connected to the source 15, which is in turn connected tothe transmitting horn 16. It has been found that a minimum of six passesof the microwave energy through the sample 17 is necessary in order toachieve a usable signal to noise ratio. However, when the microwaveenergy is transmitted through the sample 17 a number of times, it wasfound that a high radiation loss occurred with each transmission of themicrowave energy from a separate horn. It was found that this difficultyis overcome by the use of dielectric microwave lenses which collimatethe microwave energy and reduce the losses to a tolerable level.

As shown in FIG. 6, the microwave energy transmitted from the horn 16and detected by a horn 30 passes through dielectric microwave lenses 31and 32 positioned on opposite sides of the sample 17 between the horns16 and 30. The horn 30 is connected by a wave guide to. a transmittinghorn 33 wherein the microwave energy is retransmitted to the sample 17to a receiving horn 34. Dielectric microwave lenses 35 and 36 arepositioned on opposite sides of the sample 17 between the horns 33 and34. The receiving horn 34 is connected by means of a wave guide to atransmitting horn 37 which transmits the microwave energy through thesample 17 to the receiving horn 18 which is connected to the rectifier21 and the integrator 22 to the microwave indicator 19. Dielectricmicrowave lenses 38 and 39 are positioned on opposite sides of thesample 17 between the horns 37 and 18 to again collimate the microwaveenergy and reduce the losses resulting by the retransmission of themicrowave energy through the sample 17.

Another method of obtaining adequate attenuation level is that oftransmitting the microwave energy across the full width of the sample17. Such method is illustrated in FIGS. 7 and 8 of the drawings, whereinthe transmitting horn 16 is positioned at one edge of the sample 17 andthe receiving horn 18 is positioned at the opposite edge thereof.Although this method provides adequate attenuation level, the radiationlosses can be relatively high over the large distance of measurement.This problem of radiation losses, however, is overcome by the use ofmicrowave lenses 40 and 41 positioned at opposite edges of the sample 17and adjacent respective horns l6 and 18. The lenses 40 and 41 collimatethe energy and improve the efficiency of the transmission across thedistance of the sample 17.

Transmitting the microwave energy across the full width of the sample17, however, can only provide an integrated moisture content across thewidth of the sample and is not suitable for moisture profiling. It hasbeen found, however, that the same concept can be employed whilemaintaining a capability of profiling of the moisture content across thewidth of the sample 17. This is accomplished by traversing the microwaveenergy across the width of the sample 17 at relatively shallow angleswith respect thereto. Such method is illustrated in FIG. 9 wherein thetransmitting horn 16 is positioned adjacent to and below one edge of thesample l7 and the receiving horn 18 is positioned adjacent to and abovethe opposite edge of the sample 17. In order to reduce the radiationlosses, microwave lenses 42 and 43 are positioned adjacent respectivehorns l6 and 18 for colliminating the energy and improving theefficiency of the transmission across the distance of the sample 17. Theline of transmission and reception direction between the horns l6 and 18defines a relatively shallow angle designated with the reference numeral44 with the plane of the sample 17.

With the arrangement illustrated in FIG. 9, profiling is accomplished byraising and lowering the horns 16 and 18 as indicated by the arrowsadjacent thereto on the drawing in a manner'which would maintainalignment of the microwave horns l6 and 18 and their radiated energy,but would tend to move the area of attenuation to different positions onthe sample 17.

FIG. 10 illustrates another embodiment of applicants invention which isemployed for overcoming the low loss encountered in relatively drysamples under examination. In essence, the structure illustrated in FIG.10 is a travelling wave resonator. A typical description of a travellingwave resonator may be found in the I.R.E. transactions on microwavetheory and techniques, April 1958, by L. J. Millsevic and R. 'Vautey onpage 136. A ring resonator is described in Microwave Engineering, A. F.Harvey, pages 201-202. The travelling wave resonator, like the cavitydevices, might be employed to amplify low level attenuation.

As shown in FIG. 10, the modulated source of energy from the generator15 is transmitted through a wave guide 45 to a termination 46. The waveguide 45 is connected through a directional coupler 47 to a wave guide48 which is connected at one end thereof through a phase shifter 49 tothe transmitting horn l6 and at the opposite end thereof to thereceiving horn 18. The sample 17 is mounted between the horns 16 and 18and a pair of dielectric lenses 50 and 51 are disposed on opposite sidesthereof adjacent the horns 16 and 18. A directional coupler 52 isconnected between the wave guide 48 adjacent-the receiving horn 18 and awave guide 53. The wave guide 53 is connected through the rectifier 21and integrator 22 to the microwave indicator 19.

An arrangement for changing the position of the sample 17 with respectto the horns 16 and 18 is illustrated in FIGS. 11 and 12 of thedrawings. As shown therein, the horn 16 is mounted on the support member54. A plurality of rails 55, 56 and 57 are supported between the supportmember 54 and a second support member 58. The support members 54 and 58are fixedly mounted with respect to one another. Movably mounted on therails 55, 56 and 57 is a first movable support member 59 which supportsthe sample 17 thereon and a second movable support member 60 whichsupports the receiving horn 18 thereon. The receiving horn 18 isconnected by means of a wave guide 61 to the crystal detector orrectifier 21. The arrangement illustrated in FIGS. 11 and 12 permitmovement of either the sample 17 or the receiving horn 18 with respectto the transmitting horn 16. If such movement is effected over adistance which is equal to onehalf the wavelength of the microwaveenergy, the sensitivity due to sample position discussed above iseliminated.

The broad bend frequency modulated microwave moisture measurement isapplicable to many products, such as paper, textiles, foods, etc. inaddition, however, all of the above described systems for themeasurement of moisture in a sample by the broad band frequencymodulated technique can also be employed for measuring moisture in aslurry. The technique employed by the present invention for measuringmoisture in a slurry is that of extracting from a main slurry line aportion of the slurry and distributing the contents into the form of asheet or web. This is accomplished by distributing the slurry in arelatively thin or narrow cavity provided in a material of relativelylow loss dielectric.

FIGS. 13-16 illustrated a structural part which contains a relativelythin or narrow cavity therein into which the slurry can be distributed.The illustrated structural part containing the cavity therein can besupported between the microwave horns l6 and 18 for determining themoisture content of the slurry. As shown in FIGS. 13-16 a slurrydistributor 65 is formed of a pair of plates 66 and 67 made of a lowloss dielectric material. A relatively wide and shallow channel 68 isformed in one surface of the plate 67 which is joined by beveledportions 69 and 70 to an inlet 71 and an outlet 72 respectively. Ribs 73extend upwardly from a bottom surface of the channel 68 and serve todisperse the slurry equally throughout the width of the channel 68. Theplate 66 also includes a pair of beveled portion 74 and 75 which joinwith the inlet 71 and the outlet 72 respectively. When the channeledsurfaces of the plates 66 and 67 are abutted with one another and theplates are secured together in such relationship, a relatively thinchamber 76 is formed between the inlet 71 and the outlet 72 fordispersing the slurry passing therethrough into a relatively thin sheet.

It will be understood that variations and modifications may be effectedwithout departing from the spirit and scope of the novel concepts ofthis invention.

The invention claimed is:

1. Apparatus for measuring the amount of moisture in a sample in theform of a web, comprising:

a source of microwave energy;

a microwave energy radiating element connected to said source andpositioned adjacent one edge of said web to impinge microwave energy onone side of said web;

a microwave energy receiving element positioned adjacent the other edgeof said web to receive microwave energy emanating from the other side ofthe sample and aligned with said radiating element so that they arelaterally offset from each other relative to the web;

a microwave energy indicator connected to said receiving element andsaid sample being in the form of a web with its transverse dimensionlying generally in a plane which makes an acute angle with a lineextending between said radiating and receiving elements such that energyenters generally from one side of the web adjacent one edge and exitsfrom the other side of the web adjacent the other edge; means forcontinuously modulating the source of microwave energy so as to changethe energy transmission path through the sample by at least one-halfwavelength of the microwave energy;

at least one microwave collimating lens mounted between said radiatingelement and said receiving element; and

scanning means for moving said radiating and receiving elements togetherin synchronism relative to said web while maintaining alignment of saidradiating and receiving element so as to move the area of attenuationtransversely back and forth across said web.

1. Apparatus for measuring the amount of moisture in a sample in theform of a web, comprising: a source of microwave energy; a microwaveenergy radiating element connected to said source and positionedadjacent one edge of said web to impinge microwave energy on one side ofsaid web; a microwave energy receiving element positioned adjacent theother edge of said web to receive microwave energy emanating from theother side of the sample and aligned with said radiating element so thatthey are laterally offset from each other relative to the web; amicrowave energy indicator connected to said receiving element and saidsample being in the form of a web with its transverse dimension lyinggenerally in a plane which makes an acute angle with a line extendingbetween said radiating and receiving elements such that energy entersgenerally from one side of the web adjacent one edge and exits from theother side of the web adjacent the other edge; means for continuouslymodulating the source of microwave energy so as to change the energytransmission path through the sample by at least one-half wavelength ofthe microwave energy; at least one microwave collimating lens mountedbetween said radiating element and said receiving element; and scanningmeans for moving said radiating and receiving elements together insynchronism relative to said web while maintaining alignment of saidradiating and receiving element so as to move the area of attenuationtransversely back and forth across said web.